The preparation of said esters by the direct esterification of the fatty acid with the hydroxyalkylsulfonate and said amides by the amidification of the fatty acid with the aminoalkylsulfonate has presented difficulties because of the high temperature required to obtain suitable conversion. At the temperatures required for the direct esterification and amidification reactions, usually in the range of 200.degree. to 250.degree. C., the hot reaction product rapidly loses activity and degrades in color. Various methods are taught in the art to avoid loss in activity and color degradation of the reaction product.
Several patents teach the desirability of accelerating the reaction. Sundberg in U.S. Pat. No. 2,857,370 teaches the use of a boron-containing compound as a catalyst at reduced pressure or in an inert atmosphere. Anderson et al. in U.S. Pat. No. 2,923,724 disclose the use of a phosphorus containing compound such as phosphoric acid or phosphate as an accelerator. In U.S. Pat. No. 3,151,136, Koczorowski et al. teach that quantitative yields may be obtained at relatively low temperatures by using hydroxyalkylsulfonic acid which is substantially free from its salts, while operating at reduced pressure. The reaction product in this case must be neutralized to obtain the desired metal salt, introducing a further step. Zinc and zirconium salts are disclosed as catalysts for the esterification reaction by Cahn in U.S. Pat. No. 3,320,292 and U.S. Pat. No. 3,383,396, respectively.
A number of prior art patents teach the use of modifications of the fatty acid to improve the reactivity. For example, Schenck in U.S. Pat. No. 2,898,352 teaches the use of a mixed borate-fatty acid anhydride. This patent further teaches that the resulting borax may be removed from the reaction product by filtration of the molten product or by solvent extraction, using either organic solvents such as hydrocarbons, alcohols or esters to remove the fatty acid isethionic acid esters or aqueous extractions to remove the borax and sodium isethionate. Wrigley et al. in U.S. Pat. No. 3,745,181 describe the use of isopropanol fatty esters to react with hydroxyalkylsulfonate salts.
Several of the patents already mentioned also teach the desirability of maintaining a nitrogen atmosphere in order to avoid oxidation of the reaction product and also the use of reduced pressure to permit the removal of water formed during condensation at a lower temperature.
A number of patents teach a method by which the reaction product is purified so as to remove the unreacted fatty acid, sulfonate or mixture thereof that is typically present. McCrimlisk in U.S. Pat. No. 3,420,858 teaches the removal of lower fatty acids by a two-stage vacuum stripping, in which higher fatty acids are added to the reaction mixture after some of the lower acids have been removed, in order to maintain fluidity and to make possible the further removal of the lower fatty acids. Molteni in U.S. Pat. No. Patent Re. 23,823 uses an excess of sodium isethionate in his reaction and removes the excess after the esterification has taken place by dispersing the product in water, evaporating and precipitating out the desired fatty acid ester. Russell et al. in U.S. Pat. No. 2,303,582, Potter in U.S. Pat. No. 2,307,953, and Russell in U.S. Pat. No. 2,316,719 all describe methods for separating inorganic salts from organic sulfonates or sulfates by forming two-phase liquid systems in which the inorganic salt is in aqueous solution and the organic compound is dissolved in an organic solvent, which may be an alcohol such as isopropanol. The aqueous layer is drawn off to remove the inorganic salt. Landy in U.S. Pat. No. 3,880,897 describes a process in which a hydroxyalkyl sulfonate is reacted with a fatty acid halide in anhydrous dialkyl ketone. When the reaction is complete, the mixture is cooled and the insoluble ester is filtered from the dialkyl ketone solvent, washed and dried.
Holt et al. in U.S. Pat. No. 3,429,136 teach that degradation of the hot reaction product may be avoided by injecting cold water into the hot crude condensate to cool the mass below a temperature at which rapid discoloration would occur. A disadvantage of this method is that the addition of water can lead to an undesirable hydrolysis side reaction. Login et al. in U.S. Pat. No. 4,515,721 describe immersion of hot crude fatty acid ester in a liquid such as an alcohol solvent that is at a temperature lower than the crude reaction mixture to effect cooling of the reaction mixture. Cooling by this method requires the use of a liquid in which the ester is substantially insoluble and the unreacted fatty acid is soluble. A slurry is formed in which the solid phase comprises relatively pure ester and the liquid phase comprises the cooling liquid and unreacted fatty acid. The solid phase is thereafter separated from the liquid phase of the slurry, typically by filtration or by centrifugation. The filtrate is typically distilled to recover free fatty acid and cooling liquid. Hence, a disadvantage of this method is that it requires several process steps to cool and isolate the reaction product.
Urban et al. in U.S. Pat. No. 4,536,338 describe the use of an alkaline quenching material to neutralize the acid catalyst, thereby reducing or eliminating darkening and deterioration of the reaction product caused by severe stripping conditions.
All of the above mentioned citations as well as any other citations noted hereinbelow are understood to be incorporated by reference in toto into this disclosure.